The heat exchangers provided in the exterior and interior air conditioner units consist of a multi-folded refrigerant pipe which goes through a large number of fins parallelly disposed at a proper inter-fin distance, and these fins and pipe are fixed together.
The inter-fin gap is defined by the height of a collar protruding from the plate form fin material, and therefore the accuracy and performance of said heat exchanger depends largely on the accuracy of the collar height and the finish of its end portion. When a larger inter-fin distance has to be provided, a higher collar height is naturally required, and this will be accompanied by an increased difficulty to in attaining the required accuracy and performance.
As shown by Japanese Patent Publication Sho 54-31754, for example, the conventional collar forming process has been comprised of a first process consisting of forming a small diameter hole which is, at the same time, succeeded by the formation of a vertical cylinder having a diameter smaller than the required collar diameter on a specified part of a plate material, and a second process consisting of ironing and burring processes simultaneously performed by using a punch having a diameter matched to a desired collar diameter.
The details of the process are further explained by referring to FIGS. 3A-3D of the present application where a small diameter hole 33 is first formed in the material 31 by using a punch 32 to punch out the small diameter hole, and after this process, a punch 35 matched to a die 34 is inserted into said die 34 to form a small vertical cylinder 36 on said material 31 (see FIGS. 3(A) and 3(B)). Said material 31 is then placed below another die 37 in the succeeding process, and said small vertical cylinder 36 is formed into a desired collar 39 by using a punch 38 matched to said die 37 (see FIGS. 3(C) and 3(D)).
However, the process as described above is divided into a first part by which a vertical cylinder having a smaller diameter than the desired collar diameter is formed, and a second part by which the hole and collar of a desired diameter is formed, and therefore, it is impossible to accomplish a perfect coincidence between the center of the vertical cylinder having a small diameter hole formed by said first part and the center of the desired collar formed by said second part, and thus, said ironing and burring processes have to be carried out while allowing for a small discrepancy, and this causes problems such as the breakage of the collar and uneven collar height due to the discrepancy between two machining positions (see FIGS. 4(A) and 4(B)). Moreover, cracks are often generated at the top edge of the collar since the tensile stress at the time of hole expansion is concentrated on the edge of said small diameter hole since the material is protruded over the rim of said small diameter hole, and a further expansion of hole is performed in said second part (see FIG. 4(C)). Because of the independent first and second machining process parts separate tools have to be prepared, and this causes increased costs. And moreover, when a higher collar height is desired, a third part by which the burring process has to be further extended, has to be provided.